The present invention relates generally to the implantable hearing aids and more particularly to implantable hearing aids for direct coupling to the middle ear. The invention may be directly coupled to the cochlea via the stapes or to other elements within the ossicular chain.
The implantable hearing aid is intended to help a specific class of patient for which conventional hearing aids are inadequate. These patients have severe hearing impairments and require excessive amplification that is limited by acoustic feedback and sound distortion. Patients have reported an increased clarity of sound and were able to identify speech equal to or better than that reported with conventional aids by using amplified sound directly coupled to the stapes.
The performance of conventional hearing aids has improved markedly over the past ten years. However, there remains a significant population of hearing impaired patients for whom these aids are inadequate. A previously estimated population of approximately one million in the United States alone have hearing impairments that are characterized by severe hearing loss and a need for high levels of sound amplification. The high power required by these patients is difficult to achieve with the small, inconspicuous receivers used in hearing aids because of the significant mismatch between the acoustic impedance of the receiver and the ear.
Problems with these aids include: (1) inefficient energy conversion resulting in excessive power consumption and short battery life, (2) acoustic feedback resulting in oscillation and squeal, and (3) distortion of the signal from a variety of factors resulting in reduced clarity of sound and reduced speech intelligibility.
Due to impedance mismatch, conventional aids are inefficient. It has been documented that at a sound pressure of 100 dB SPL the acoustic power absorbed by the ear is about 0.1 microwatt. The corresponding electrical power supplied to the hearing aid receiver operating at this level is about 0.3 milliwatts (300 microwatts). The conversion efficiency is less than 0.3 percent. In contrast, the conversion efficiency of a well designed acoustic horn and driver is between 10 percent and 50 percent. This sizable discrepancy in efficiency between the hearing aid and horn driver is a direct consequence of the relative impedances of the transducer and the acoustic impedance of the load on the transducer. By coupling directly to the stapes, a better impedance match is achieved and the system can be made more efficient. A tenfold improvement in efficiency would result directly in extending the battery life of the aid.
A second problem with conventional high-power hearing aids is acoustic feedback. In order to isolate the output of the aid from the input microphone, patients are required to wear tightly-fitting earmolds. These tightly-fitting earmolds are uncomfortable for the patient and the complete occlusion of the ear canal causes an unpleasant sensation and initially makes the patient's own voice sound unnatural. In addition, the amount of isolation that can be achieved with a closed earmold is limited by the acoustic properties of available materials and even the best fitting earmold will lose its seal as a result of jaw motion and external ear movement. These problems can be solved with the use of direct vibratory stimulation of the stapes. Although some sound will radiate from the cochlea, it will be greatly attenuated because of the small impedance of the ear canal versus the higher acoustic impedance of the transducer and fluid-filled cochlea.
A third problem with conventional hearing aids is the distortion produced by the receiver and the middle ear at high sound pressures. Distortion results in a loss of clarity of sound and a reduction in speech intelligibility for the patient. It has been illustrated that a speech hearing loss due to attenuation can be corrected with amplification, whereas a speech hearing loss due to distortion cannot. The distortion inherent in an abnormally functioning inner ear makes it impossible for the patient to recognize individual phonemes of speech with 100 percent accuracy under quiet listening conditions. When this inherent distortion is coupled with that caused by conventional hearing aids (i.e., acoustic feedback, acoustic resonances and antiresonances due to receiver and connective tubing and middle ear distortion) the sum of the individual sources of distortion on speech intelligibility is similar to that of decreasing the speech-to-noise ratio at the input to the hearing aid. Since a normal hearing individual has only a margin of 8 dB in a typical noisy environment such as a busy department store or restaurant, an equivalent loss of 8 dB due to distortion can become a major handicap. As previously mentioned, with middle ear vibrators, patients have reported that the sound perceived via direct stimulation of the stapes is clearer and less distorted when compared with the sound produced by conventional aids. From these results it appears that driving the stapes directly can eliminate much of the distortion of conventional aids.
Concerning utility, an important aspect of hearing aid design is its utility for the wearer which includes factors such as patient comfort, convenience of use, sound quality and aesthetics. Utility of the design plays an important role in patient acceptance and must be included in the evaluation of a device. In the past, the emphasis on whether an aid is satisfactory has been determined primarily by speech intelligibility testing. Previous evidence implies that if a patient is given a choice, he will prefer to operate the aid (initially) at a setting that provides better sound quality rather than maximum intelligibility. In the same way, patients are likely to prefer to wear a device that is more comfortable and less conspicuous. These issues of utility can be best served by an implantable hearing aid. However, presently available microphones (acoustical-to-electrical transducers) and receivers (electrical-to-acoustical transducers) are too large and ill-suited for placement within the middle ear.
The present invention makes it possible to use existing transducers, without significant modification to the middle ear while retaining the advantages of implanted middle ear assistance devices. The present invention provides an acoustic coupler which is preferably hermetically sealed for direct insertion into the middle ear cavity. The acoustic coupler is sized to fit within the middle ear cavity without significant surgical alteration of the cavity. It can be attached to a microphone located remote from the middle ear cavity for acoustically coupling the microphone with the malleus or the tympanic membrane. The coupler may also be used in reverse when attached to a receiver to act as a vibrator for causing mechanical vibration of the stapes.
The acoustic coupler comprises a chamber-forming member across which a compliant diaphragm or membrane is attached. Connective tubing couples the chamber with a selected electroacoustic transducer (e.g., microphone or receiver). The connective tubing is preferably acoustically matched so that a substantially constant acoustical impedance is maintained. The presently preferred connective tubing system includes a tuning portion forming a terminated or closed end, resonant at even harmonics and an open end, resonant at odd harmonics. The resultant acoustical coupler is suitable for direct placement within the middle ear cavity and serves an acoustical coupling for transmission of acoustical energy between the coupler diaphragm at one end and the electroacoustic transducer at the other. The coupler diaphragm may be physically coupled to the handle of the malleus by a wire hook or it may be positioned to acoustically couple with the tympanic membrane when used as a microphone. When used as a transmitter or vibrator the diaphragm can be physically attached to the stapes via an intermediate wafer adhered to the membrane and also attached to the stapes. Preferably the wafer is porous such as ceramic and becomes fused to the stapes by tissue growth.